Re: [PATCH 1/2] rwsem: add rwsem_is_contended

[Josef Bacik]

On 01/13/2014 02:02 PM, Ingo Molnar wrote:
> * Josef Bacik <jbacik@xxxxxx> wrote:
>
> > From: Josef Bacik <jbacik@xxxxxxxxxxxx>
> >
> > Btrfs needs a simple way to know if it needs to let go of it's read lock on a
> > rwsem.  Introduce rwsem_is_contended to check to see if there are any waiters on
> > this rwsem currently.  This is just a hueristic, it is meant to be light and not
> > 100% accurate and called by somebody already holding on to the rwsem in either
> > read or write.  Thanks,
> >
> > Signed-off-by: Josef Bacik <jbacik@xxxxxxxxxxxx>
> > ---
> >   include/linux/rwsem.h | 11 +++++++++++
> >   1 file changed, 11 insertions(+)
> >
> > diff --git a/include/linux/rwsem.h b/include/linux/rwsem.h
> > index 0616ffe..03f3b05 100644
> > --- a/include/linux/rwsem.h
> > +++ b/include/linux/rwsem.h
> > @@ -75,6 +75,17 @@ do {								\
> >   } while (0)
> >   
> >   /*
> > + * This is the same regardless of which rwsem implementation that is being used.
> > + * It is just a heuristic meant to be called by somebody alreadying holding the
> > + * rwsem to see if somebody from an incompatible type is wanting access to the
> > + * lock.
> > + */
> > +static inline int rwsem_is_contended(struct rw_semaphore *sem)
> > +{
> > +	return !list_empty(&sem->wait_list);
> > +}
> > +
> > +/*
> >    * lock for reading
> >    */
> >   extern void down_read(struct rw_semaphore *sem);
> Looks good to me. To make it easier to merge 2/2 feel free to add this
> to the BTRFS tree, with my Acked-by:
>
>    Acked-by: Ingo Molnar <mingo@xxxxxxxxxx>
Great thanks Ingo,

Josef
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